Pavel Škarvada

Detection and Localization of Defects in Monocrystalline Silicon Solar Cell

Near-surface defects in solar cell wafer have undesirable influence upon device properties, as its efficiency and lifetime. When reverse-bias voltage is applied to the wafer, a magnitude of electric signals from defects can be measured electronically, but the localization of defects is difficult using classical optical far-field methods. Therefore, the paper introduces a novel combination of electric and optical methods showing promise of being useful in detection and localization of defects with resolution of 250 nm using near-field nondestructive characterization techniques. The results of mapped topography, local surface reflection, and local light to electric energy conversion measurement in areas with small defects strongly support the development and further evaluation of the technique.

Contact quality analysis and noise sources determination of CdZnTe-based high-energy photon detectors

Experimental studies of the transport and noise characteristics of a cadmium-zinc-telluride (CdZnTe) crystal with symmetric gold contacts and a guard ring electrode have been carried out. The current–voltage (IV) characteristics and the noise spectral density were measured at room temperature in the dark. The sample with a disconnected guard ring electrode showed symmetric characteristics for both bias voltage polarities. The shape IV characteristics indicated the presence of carrier injection, leading to IV characteristics nonlinearity. The semiconductor surface has been identified as the main noise and leakage current source. After connecting the guard ring electrode, the leakage currents were suppressed by two orders and the noise spectral density decreased by five orders. In the case of the connected guard ring electrode, the asymmetry of IV characteristics was observed. The contact with worse rectifying properties had a higher contribution of noise to the detector system. Increasing bias voltage causes steeper detector additive noise growth when the guard ring electrode is disconnected.

Study of electric field distribution and low frequency noise of CdZnTe radiation detectors

Polarization phenomena in a metal-semiconductor-metal (M-S-M) structure of metallic Schottky contacts deposited on CdZnTe radiation detectors were studied. We evaluate the distribution of the electric field along the biased M-S-M structure by Pockels measurements. The results show that almost all the electric field is developed across the depletion layer of the reverse-biased contact. The noise measurements of the CdZnTe detectors studied show that the dominant noise is 1/fm noise. The 1/fm noise, with the parameter m close to one, is present at frequencies below 100 Hz and its bandwidth decreases in the course of the polarization process. At higher frequencies, we observed an increase of the m parameter to 2, which indicates a strengthened effect of the generation-recombination processes. In the frequency band of dominating 1/fm = 1 noise, the increase of magnitude of the noise spectral density was proportional to the power of 6, in relation to the current through the detector. This high value is explained as a result of a screening effect of the space charge buildup during the polarization.

Nanooptics of locally induced photocurrent in monocrystalline Si solar cells

This paper presents the results of our experimental study of high resolution map of induced photocurrent in monocrystalline silicon solar cells. Photovoltaic solar cells are evaluated by Near-field Optical Beam Induced photocurrent (NOBIC), as well as by Scanning Near-field Optical Microscope (SNOM) topography and reflection. The correlation between reflection and transport characteristic indicates possibility of this diagnostic tool. Therefore the SNOM and NOBIC represent the coupling of very useful methods to provide a non-destructive local characterization on silicon semiconductor solar cells.

AFM imaging of natural optical structures

The colors of some living organisms assosiated with the surface structure. Irridesence butterfly wings is an example of such coloration. Optical effects such as interference, diffraction, polarization are responsible for physical colors appearance. Alongside with amazing beauty this structure represent interest for design of optical devices. Here we report the results of morphology investigation by atomic force microscopy. The difference in surface structure of black and blue wings areas is clearly observed. It explains the angle dependence of the wing blue color, since these micrometer and sub-micrometer quasiperiodical structures could control the light propagation, absorption and reflection.

Near-Field Study of Hot Spot Photoluminescence Decay in ZnS:Mn Nanoparticles

The local spatial distribution of photoluminescence due to the creation of hot luminescence centers was measured in the optical near-field by Scanning near-field optical microscope at emission peaks of materials (λ =595nm), which is due to the luminescence of Mn2+ in ZnS. The excitation bandgap of ZnS forms exitons, and these excitons get the center of Mn2+ through nonradiation dominates, by means of transition of 4T1 – 6A1 luminescence. This spectrum is evidence that Mn2+ has been incorporated into the ZnS nanoparticles. In comparison with the bulk ZnS:Mn phosphors these nanoparticles have clearly higher luminescent efficiency with its luminescent decay time at least 4 orders of magnitude slower. It means that the oscillator intensity of luminescent centers in ZnS:Mn nanocrystal enhances at least 4 orders of magnitude than that in corresponding bulk ZnS:Mn.

Detection of microstructural defects in chalcopyrite Cu(In,Ga)Se2 solar cells by spectrally-filtered electroluminescence

The aim of this research is to detect and localize microstructural defects by using an electrically excited light emission from a forward/reverse-bias stressed pn-junction in thin-film Cu(In; Ga)Se2 solar cells with metal wrap through architecture. A different origin of the local light emission from intrinsic/extrinsic imperfections in these chalcopyrite-based solar cells can be distinguished by a spectrally-filtered electroluminescence mapping. After a light emission mapping and localization of the defects in a macro scale is performed a micro scale exploration of the solar cell surface by a scanning electron microscope which follows the particular defects obtained by an electroluminescence. In particular, these macroscopic/microscopic examinations are performed independently, then the searching of the corresponding defects in the micro scale is rather difficult due to a diffused light emission obtained from the macro scale localization. Some of the defects accompanied by a highly intense light emission very often lead to a strong local overheating. Therefore, the lock-in infrared thermography is also performed along with an electroluminescence mapping.

Multiscale experimental characterization of solar cell defects

The search for alternative sources of renewable energy, including novel photovoltaics structures, is one of the principal tasks of 21th century development. In the field of photovoltaics there are three generations of solar cells of different structures going from monocrystalline silicon through thin-films to hybrid and organic cells, moreover using nanostructure details. Due to the diversity of these structures, their complex study requires the multiscale interpretations which common core includes an integrated approach bridging not only the length scales from macroscale to the atomistic, but also multispectral investigation under different working temperatures. The multiscale study is generally applied to theoretical aspects, but is also applied to experimental characterization. We investigate multiscale aspects of electrical, optical and thermal properties of solar cells under illumination and in dark conditions when an external bias is applied. We present the results of a research of the micron and sub-micron defects in a crystalline solar cell structure utilizing scanning probe microscopy and electric noise measurement.

Investigation of adsorption-desorption phenomenon by using current fluctuations of amperometric NO 2 gas sensor

Nitrogen dioxide (NO2) is a highly toxic gas harmful to the environment even at low concentrations. To overcome limitations of standard solid NO2 sensors based on inorganic materials, a fully printed sensor with solid polymer electrolytes (SPE) was developed in Regional Innovation Centre for Electrical engineering in Czech Republic. The amperometric sensor is based on a semi-planar three-electrode topology (reference, working and counter electrode) and solid polymer electrolyte. This paper focuses on adsorption-desorption phenomenon by studying current fluctuations and current mean value of amperometric NO2 gas sensor during its exposition to concentration cycle. Limits of NO2 concentration are from 0 to 6 ppm. Background noise and thermal noise are only apparent for zero concentration, while, noise 1/f becomes main component of current fluctuations for higher NO2 concentrations.

Realization of microscale detection and localization of low light emitting spots in monocrystalline silicon solar cells

We report on detection and localization of imperfections in silicon solar cell bulk and surface with sub-micrometer resolution. To obtain this resolution, a family of imaging techniques including SNOM, SEM and AFM is often separately used for this purpose. In this paper we combine several of these proximal methods together, because each of them brings complimentary information about the imperfection. First, we note that SNOM images often contain distortions due to the interaction of the probe tip and sample. Therefore, we look for the possibility to circumvent this weakness and obtain more realistic images. In our experiments, we take advantage of the fact that defects or imperfections in silicon solar cell structures under reverse-bias voltage exhibit microscale low light emitting spots, and we apply an improved SNOM measurement to localize these spots. As a result, this system allows a localization and measurement of low light emission on microscale. Consequently, the size and shape of imperfections can also be determined.